1. Field of the Invention
The present invention relates to a gate turn-off thyristor (referred to as a " GTO thyristor" hereinafter) which consists of a four-layer pnpn structure and which can be turned off by the current removed from a gate electrode disposed on one base layer.
2. Description of the Prior Art
FIG. 3 is a cross sectional view showing a portion of a conventional GTO thyristor (the portion being referred to as a "segment" hereinafter) which has a four-layer structure consisting of a p-type emitter layer 1, an n-type base layer 2, a p-type base layer 3 and an n-type emitter layer 4. A first main electrode, for example, an anode electrode 5 is applied to the p-type emitter layer 1. A gate electrode 6 is applied to the p-type base layer 3. A second electrode, for example, a cathode electrode 7 is applied to the n-type emitter layer 4. The n-type emitter layer 4 is in the form of a strip or rectangular island.
FIG. 4 is a plan view showing a GTO thyristor device having four segments, one of which is shown in FIG. 3, when viewed from the upper side of the cathode electrode 7. In this device, the anodes 5 of the four segments are connected in parallel and the cathodes 7 are also connected in parallel, while the gate electrodes 6 are connected in common. A controllable on-state current, that is, a turn-off current, which is one of important features of a GTO thyristor, is realized by removing a gate current from the gate electrode 6 arranged and connected in common.
In an actual GTO thyristor in which a plurality of segments are arranged in parallel with each other, the electrical characteristics of the respective segments, such as on-state voltages and gate impedance values, vary one from another, so that in the case of turning off the GTO thyristor device, the operations of removing the gate currents from the gate electrodes 6 of the respective segments are not uniform among the segments. At the last stage of the turning off operation of the GTO thyristor device, a specific segment, for example, a segment where an on-state voltage is low or a gate impedance is high, may experience a "turn-off-failure" due to the concentration of current to the specific segment. By the concentration of the current to the specific segment, an increase of the controllable on-state current of the GTO thyristor is prevented. In the case of the formation of each layer by a conventional diffusion process and a lifetime-killer injection process, there is a limited capability of making the electrical characteristics of all segments uniform, so that a remarkable increase of the controllable on-state current cannot be expected.